[ADDED] the inlog screen and a data singleton for the client

Client/App.xaml.cs

@@ -1,4 +1,5 @@
-using System;
+using Client.Views;
+using System;
 using System.Collections.Generic;
 using System.Configuration;
 using System.Data;
@@ -17,9 +18,9 @@ namespace Client
         protected override void OnStartup(StartupEventArgs e)
         {
             base.OnStartup(e);
-            MainWindow startWindow = new MainWindow();
-            ViewModel VM = new ViewModel();
-            startWindow.DataContext = VM;
+            LoginScreen startWindow = new LoginScreen();
+            //ViewModel VM = new ViewModel();
+            //startWindow.DataContext = VM;
             startWindow.Show();
         }
 

Client/ClientData.cs

@@ -0,0 +1,59 @@
+using SharedClientServer;
+using System;
+using System.Collections.Generic;
+using System.Data;
+using System.Text;
+
+namespace Client
+{
+    class ClientData
+    {
+        private static ClientData _instance;
+        private static readonly object padlock = new object();
+
+        public static ClientData Instance
+        {
+            get
+            {
+                lock (padlock)
+                {
+                    if (_instance == null)
+                    {
+                        _instance = new ClientData();
+                    }
+                    return _instance;
+                }
+            }
+        }
+
+
+        private User _user;
+        private Client _client;
+        private Lobby _lobby;
+
+        private ClientData()
+        {
+
+        }
+
+
+        public User User
+        {
+            get { return _user; }
+            set { _user = value; }
+        }
+
+        public Client Client
+        {
+            get { return _client; }
+            set { _client = value; }
+        }
+
+        public Lobby Lobby
+        {
+            get { return _lobby; }
+            set { _lobby = value; }
+        }
+
+    }
+}

Client/ViewModels/ViewModel.cs

@@ -18,7 +18,7 @@ namespace Client
             ButtonCommand = new RelayCommand(() =>
             {
                 Client client = new Client();
-            });
+            }, true);
 
             _lobbies = new List<Lobby>();
 

Client/Views/GameWindow.xaml.cs

@@ -18,7 +18,7 @@ namespace Client.Views
     /// </summary>
     public partial class GameWindow : Window
     {
-
+        ClientData data = ClientData.Instance;
         public GameWindow()
         {
             DataContext = new ViewModelGame();
@@ -110,7 +110,7 @@ namespace Client.Views
          */
         private void WriteToChat(string message)
         {
-            string user = "Monkey"; 
+            string user = data.User.Username; 
             SentMessage.AppendText($"{user}: {message}\n");
         }
     }

Client/Views/LoginScreen.xaml

@@ -0,0 +1,29 @@
+<Window x:Class="Client.Views.LoginScreen"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:local="clr-namespace:Client.Views"
+        mc:Ignorable="d"
+        Title="LoginScreen" Height="450" Width="800">
+    <Grid>
+        <Grid.RowDefinitions>
+            <RowDefinition Height="60"/>
+            <RowDefinition Height="50"/>
+            <RowDefinition Height="30"/>
+        </Grid.RowDefinitions>
+
+        <Grid.ColumnDefinitions>
+            <ColumnDefinition />
+            <ColumnDefinition />
+        </Grid.ColumnDefinitions>
+
+        <Label Grid.Row="0" FontSize="40" Content="Welcome to Scrubl.io"/>
+        <Label Grid.Row="1" FontSize="30" Content="Enter a username:"/>
+        <Label Grid.Row="2" FontSize="15" Content="(max amount of characters for the username is 10)"/>
+
+        <TextBox Name="usernameTextbox" Grid.Row="1" Grid.Column="1" MaxLength="10" FontSize="30" VerticalAlignment="Center" HorizontalAlignment="Left" Width="250"/>
+        <Button Content="ENTER" Grid.Column="1" Grid.Row="1" VerticalAlignment="Center" HorizontalAlignment="Right" Width="100" Height="40" Click="Button_EnterUsername"/>
+
+    </Grid>
+</Window>

Client/Views/LoginScreen.xaml.cs

@@ -0,0 +1,40 @@
+using SharedClientServer;
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Data;
+using System.Windows.Documents;
+using System.Windows.Input;
+using System.Windows.Media;
+using System.Windows.Media.Imaging;
+using System.Windows.Shapes;
+
+namespace Client.Views
+{
+    /// <summary>
+    /// Interaction logic for LoginScreen.xaml
+    /// </summary>
+    public partial class LoginScreen : Window
+    {
+        ClientData data = ClientData.Instance;
+        public LoginScreen()
+        {
+            InitializeComponent();
+        }
+
+        private void Button_EnterUsername(object sender, RoutedEventArgs e)
+        {
+            User user = new User(usernameTextbox.Text, 0, false);
+            Client client = new Client();
+
+            data.User = user;
+            data.Client = client;
+
+            MainWindow startWindow = new MainWindow();
+            startWindow.Show();
+            this.Close();
+        }
+    }
+}

Client/Views/MainWindow.xaml.cs

@@ -21,9 +21,13 @@ namespace Client
     /// </summary>
     public partial class MainWindow : Window
     {
+        ClientData data = ClientData.Instance;
         public MainWindow()
         {
+            this.DataContext = new ViewModel();
             InitializeComponent();
+
+            usernameTextbox.Text = data.User.Username;
         }
 
         private void Button_Click(object sender, RoutedEventArgs e)

Server/Models/ServerCommunication.cs

@@ -132,7 +132,7 @@ namespace Server.Models
                     {
                         foreach(ServerClient sc in serverClients)
                         {
-                            if (sc.Username == username)
+                            if (sc.Username == user.Username)
                             {
                                 serverClientsInlobbies[l].Add(sc);
                                 break;

SharedClientServer/Lobby.cs

@@ -41,7 +41,7 @@ namespace Client
             succes = false;
             if (_users.Count < _maxPlayers)
             {
-                _users.Add(new User(username, 0));
+                _users.Add(new User(username, 0, false));
                 succes = true;
             }
         }